Process of making multicomponent or asymmetric gas separation membranes

ABSTRACT

A process for making gas separation membranes having enhanced selectivity for a mixture of gases is disclosed. The membranes may be asymmetric or multicomponent. The membranes surprisingly provide selectivity for gases in a mixture that approaches the relative selectivity of the single gas components. Preferably the membrane provides selectivity for a mixture of gases which is at least 65%, preferably 80%, of the relative selectivity of the corresponding single gases.

FIELD OF THE INVENTION

The present invention relates to composite or asymmetric gas separationmembranes, particularly, gas separation membranes in which theselectivity of gases in gas mixtures approaches their correspondingsingle gas selectivity; and a process for the fabrication of suchmembranes.

BACKGROUND OF THE INVENTION

The separation of one or more gases from a complex multicomponentmixture of gases is necessary in a large number of industries. Suchseparations currently are undertaken commercially by processes such ascryogenics, pressure swing adsorption and membrane separations. Incertain types of gas separations, membrane separations have been foundto be economically more viable than other processes.

In a pressure driven gas membrane separation process, one side of thegas separation membrane is contacted with a complex multicomponent gasmixture and certain of the gases of the mixture permeate through themembrane faster than the other gases. Gas separation membranes therebyallow some gases to permeate through them while serving as a barrier toother gases in a relative sense. The relative gas permeation ratethrough the membrane is a property of the membrane material compositionand its morphology. It has been suggested in the prior art that theintrinsic permeability of a polymer membrane is a combination of gasdiffusion through the membrane, controlled in part by the packing andmolecular free volume of the material, and gas solubility within thematerial. Selectivity is determined by dividing the permeabilities oftwo gases being separated by a material. It is also highly desirable toform defect-free dense separating layers in order to retain high gasselectivity.

In gas separations, it is also advantageous to use membranes whichpossess the desired properties of selectivity, flux, and mechanicalstrength to withstand prolonged operation at high temperatures andpressures. Furthermore, in order for gas separations to be commerciallyviable, it is advantageous to use membranes that can be manufactured inlarge quantities at high product quality, and which can be inexpensivelyassembled into a permeator.

The preparation of commercially viable gas separation membranes has beengreatly simplified with asymmetric membranes. Asymmetric membranes areprepared by the precipitation of polymer solutions in solvent-misciblenonsolvents. Such membranes are typified by a dense separating layersupported on an anisotropic substrate of a graded porosity and aregenerally prepared in one step. Examples of such membranes and theirmethods of manufacture are disclosed in U.S. Pat. Nos. 4,113,628;4,378,324; 4,460,526; 4,474,662; 4,485,056; and 4,512,893. U.S. Pat. No.4,717,394 shows preparation of asymmetric separation membranes fromselected polyimides.

Composite gas separation membranes typically have a dense separatinglayer on a preformed microporous substrate. The separating layer and thesubstrate are usually different in composition. Examples of suchmembranes and their methods of manufacture are disclosed in U.S. Pat.Nos. 4,664,669; 4,689,267; 4,741,829; 2,947,687; 2,953,502; 3,616,607;4,714,481; 4,602,922; 2,970,106; 2,960,462; and 4,713,292.

Composite gas separation membranes have evolved to a structure of anultrathin, dense separating layer supported on an anisotropic,microporous substrate. These composite membrane structures can beprepared by laminating a preformed ultrathin dense separating layer ontop of a preformed anisotropic support membrane. Examples of suchmembranes and their methods of manufacture are disclosed in U.S. Pat.Nos. 4,689,267; 4,741,829; 2,947,687; 2,953,502; 2,970,106; 4,086,310;4,132,824; 4,192,824; 4,155,793; and 4,156,597.

Composite gas separation membranes are generally prepared by multistepfabrication processes. Typically, the preparation of composite gasseparation membranes require first forming an anisotropic, poroussubstrate. This is followed by contacting the substrate with amembrane-forming solution. Examples of such methods are shown in U.S.Pat. Nos. 4,826,599; 3,648,845; and 3,508,994. U.S. Pat. No. 4,756,932shows forming composite hollow-fiber membranes by dip coating.Alternatively, composite hollow-fiber membranes may also be prepared byco-extrusion of multiple polymer solution layers, followed byprecipitation in a solvent-miscible nonsolvent.

The hollow-fiber spinning process depends on many variables which mayaffect the morphology and properties of the hollow-fiber membrane. Thesevariables include the composition of the polymer solution employed toform the fiber, the composition of fluid injected into the bore of thehollow-fiber extrudate during spinning, the temperature of thespinneret, the coagulation medium employed to treat the hollow-fiberextrudate, the temperature of the coagulation medium, the rapidity ofcoagulation of the polymer, the rate of extrusion of the fiber, takeupspeed of the fiber onto the takeup roll, and the like.

A particular problem has been observed during the use of asymmetric andcomposite membranes for the separation of gas mixtures. In particular,the selectivity of gas separation membranes is significantly poorer formixed gas separations than the corresponding ratio of the single gaspermeabilities. For example, in a polyimide gas separation membrane witha feed stream containing 90% N₂ and 10% CO₂, at room temperature theselectivity for CO₂ /N₂ may be about 20; whereas the ratio of single gaspermeability for CO₂ to the permeability for N₂ may be about 40. A needtherefore exists for a membrane and a process of manufacture whichavoids the above shortcomings of the prior art membranes and processes.The present invention is directed to improved membranes, particularlyhollow-fiber membranes and their methods of manufacture. The invention,although applicable to membranes generally, has particular utility tohollow-fiber asymmetric and composite membranes. The improvedhollow-fiber membranes are produced by varying the spinning solutionformulations and the spinning process conditions to achieve the desiredfiber morphology to provide fibers that have improved permeationproperties and mechanical strength. The fiber membranes are especiallyuseful in gas separations that require the use of high feed pressures.

SUMMARY OF THE INVENTION

The invention provides a gas separation membrane prepared by the processof extruding one or more film-forming polymer solutions to form anascent membrane, followed by precipitation to form a membrane. Themembrane may be asymmetric or composite. The film-forming polymer is notlimited and may be selected from polymers such as polysulfones,polyethersulfones, polyetherimides, polyimides or polyamides. Thenascent membrane can be optionally partially dried prior to coagulatingthe membrane in a fluid bath. The nascent membrane is quenched and thenthe remainder of the solvent may be removed to form the gas separationmembrane.

The membranes may be formed into hollow fibers, as well as shapes suchas films. The composite membranes have at least two componentscomprising a first layer material for supporting a second, separatinglayer for separating gases. The second layer can be in the form of anasymmetric membrane which contains a dense gas separating layer on theexterior surface of the membrane.

The improved gas separation membranes are produced by adjusting themorphology of the membrane. It is believed that advantageous propertiesare achieved by reducing the free volume in the dense separating layerof the membrane, which minimizes the dual mode competition for the fastand the slow gases for soprtion and transport in the glassy polymermatrix.

It is generally observed that high flux membranes possessing high freevolume in the separating structure of the membrane which are more proneto dual mode competition exhibit higher depression in the mixed gas CO₂flux and the CO₂ (10%)N₂ (90%) selectivity. This dual mode competitionis described in an article entitled Reversible Isopentane-InducedDepression of Carbon Dioxide Permeation through Polycarbonate by R. T.Chem, W. J. Koros, H.B. Hopfenberg, and V. T. Stannett; Department ofChemical Engineering, North Carolina State University, Raleigh, N.C.27650, Journal of Polymer Science, Polymer Physics Edition, Vol. 21,753-763 (1983) The depression in the mixed gas selectivity is reducedwith decreasing concentration of the slow gas in the feed mixture.

The desired membrane morphology is obtained by adjusting the spinningpolymer solution formulations and the spinning process conditions. Inparticular, advantageous properties may be obtained by increasing theweight percent of polymer in the spin dope solution, increasing thespinneret temperature, increasing the residence time of the nascentmembrane in the air gap and/or lowering the temperature of the quenchbath.

The preferred process for making a multicomponent gas separationmembrane of the invention comprises the steps of:

a. dissolving first polymer(s) in a suitable solvent to form a coresolution;

b. dissolving second polymer(s) in a solvent to form a sheath solutionhaving at least 26, preferably 27-35, most preferably 27-29, weightpercent polymer;

c. coextruding the core and sheath solutions at a temperature of greaterthan 80° C., preferably 85°-100° C., through a spinneret having at leastone hollow fiber spinning orifice to provide at least one nascentmulticomponent hollow fiber membrane;

d. drawing said nascent multicomponent hollow fiber membrane through anair gap of more than 2.0 cm, preferably more than 5 cm, most preferably7-10 cm;

e. introducing said nascent multicomponent hollow fiber membrane into acoagulation bath having a temperature of less than 25° C., preferably0°-20° C., most preferably 5°-15° C., to solidify the nascentmulticomponent hollow fiber membrane into a hollow fiber membrane.

A corresponding process may also be used to make an asymmetric gasseparation membrane.

Although it is generally known in the art that these process parametersmay be adjusted to alter the flux and selectivity of the membrane fiber,it has, surprisingly, been discovered that the selectivity of the fiberproduced by the present invention is improved such that the selectivityof gases in a mixture approaches the relative selectivity of singlegases. The preferred membrane provides selectivity for a mixture ofgases which is at least 65%, preferably 80%, most preferably at least90% of the relative selectivity of the corresponding single gases.

DETAILED DESCRIPTION OF THE INVENTION Components of the Membrane

The present invention allows for manufacture of improved multicomponentand asymmetric gas separation membranes. In manufacture of themembranes, a wide range of materials may be used as the gas separatinglayer. These materials include polyamides, polyimides, polyesters,polycarbonates, copolycarbonate esters, polyethers, polyetherketones,polyetherimides, polyethersulfones, polysulfones, polyvinylidenefluoride, polybenzimidazoles, polybenzoxazoles, polyacrylonitrile,cellulosic derivatives, polyazoaromatics, poly(2,6-dimethylphenyleneoxide), polyphenylene oxides, polyureas, polyurethanes, polyhydrazides,polyazomethines, polyacetals, cellulose acetates, cellulose nitrate,ethyl cellulose, styrene-acrylonitrile copolymers, brominatedpoly(xylylene oxide), sulfonated poly(xylylene oxide),tetrahalogen-substituted polycarbonates, tetrahalogen-substitutedpolyesters, tetrahalogen-substituted polycarbonate esters,polyquinoxaline, polyamideimides, polyamide esters, polysiloxanes,polyacetylenes, polyphosphazenes, polyethylenes, poly4-methylpentene),poly(trimethylsilylpropyne), poly(trialkylsilylacetylenes), polyureas,polyurethanes, blends thereof, copolymers thereof, substituted materialsthereof, and the like. It is further anticipated that polymerizablesubstances, that is, materials which cure to form a polymer, such asvulcanizable siloxanes and the like, may be suitable for making the gasseparation membranes of the present invention. Preferred materials forthe dense gas separating layer of multicomponent membranes includearomatic polyamide and aromatic polyimide compositions, such as thosedescribed in U.S. Pat. No. 5,085,676.

Suitable substrate layer materials for multicomponent membranes of thepresent invention may include polysulfone, polyethersulfone, polyamide,polyimide, polyetherimide, polyesters, polycarbonates, copolycarbonateesters, polyethers, polyetherketones, polyvinylidene fluoride,polybenzimidazoles, polybenzoxazoles, cellulosic derivatives,polyazoaromatics, poly2,6-dimethylphenylene oxide), polyarylene oxide,polyureas, polyurethanes, polyhydrazides, polyazomethines, celluloseacetates, cellulose nitrates, ethyl cellulose, brominated poly(xylyleneoxide), sulfonated poly(xylylene oxide), polyquinoxaline,polyamideimides, polyamide esters, blends thereof, copolymers thereof;substituted materials thereof and the like. This should not beconsidered limiting because any materials which can be fabricated intoan anisotropic substrate membrane may find utility as the substratelayer of the present invention. Preferred materials for the substratelayer include polysulfone, polyethersulfone, polyetherimide, polyimideand polyamide compositions. Especially preferred substrate materials aredescribed in U.S. Pat. No. 5,085,676.

The polymers for an asymmetric membrane and for both the substrate gasseparating layer of a multicomponent membrane should have a sufficientlyhigh molecular weight to be film forming.

Gas separation membranes of the present invention may be in the form ofvarious shapes such as flat membranes or hollow-fiber membranes. Themembrane is preferably in the form of a hollow fiber due to the surfacearea advantages available. The flat film membranes may be preparedthrough coextrusion of the polymer solutions for the separating andsupport layers to form a nascent multilayer membrane.

Fabrication of Inventive Membranes

For the purpose of illustrating the invention, the following discussionexemplifies forming multicomponent membranes with two components, thatis, a gas separating component and a substrate component. This shouldnot be considered limiting, however, because this method is useful forforming asymmetric membranes. In addition, the multicomponent membranesof the present invention may incorporate more than two component layers.The additional layers may function as gas separating layers, structurallayers, compatibilizing layers, substrate layers, layers which reduceenvironmental concerns, or combinations thereof. These additional layersmay contain the materials employed in the gas separating layer and thesubstrate layer.

The materials of each layer of the multicomponent membrane should besufficiently compatible to ensure integrity of the composite membraneduring processing or when employed in gas separations.

Multicomponent hollow fiber membranes in the form of hollow fibers maybe formed by coextrusion of the support polymer and separating polymersolutions. For example, polymer solutions for the layers may becoextruded through a multiple channel spinneret while maintaining a gaspressure or a bore fluid in the nascent hollow fiber bore to maintainthe fiber's structural integrity. Such multiple channel spinnerets havebeen described in the prior art for use in melt extrusion ofmulticomponent fibers.

Coextrusion, and the apparatus and processes therein, of polymers iswell known in the art. The improved invention for the fabrication of gasseparation membranes, however, is novel and surprising.

During fabrication of the hollow fiber membranes, the separating layeris preferably formed on the outside surface of the fiber to maximize themembrane surface area exposed to the gas. However, the separating layeralso may be formed as the inner layer of fiber. The multicomponenthollow fiber membrane of the present invention may have an outsidediameter of about 75 to 1,000 microns, preferably 100 to 350 microns,and a wall thickness of about 25 to 300 microns, preferably 25 to 75microns. Preferably, the diameter of the bore of the fiber is aboutone-half to three-quarters of the outside diameter of the fiber.

The porosity of the resultant membrane is sufficient so that the voidvolume of the membrane is within the range of 10 to 90 percent,preferably about 30 to 70 percent, based on the volume contained within.

The polymers employed in the preparation of the hollow fiber membraneshave sufficiently high molecular weight that the resultant spin dopeformulations can be extruded through a spinneret to form aself-supporting hollow fiber which can be processed in the subsequentsteps of the spinning process. Typical zero-shear viscosities of thespin dopes at 70° C. are in excess of several hundred poise, preferablyin the range of 100 to 5000 poise.

As mentioned, the spin dope formulations are extruded through aspinneret to provide hollow fiber membranes. The combination of thevolumetric rate of supply (measured in terms of cubic centimeters ofdope/unit time) of the spin dope to the spinneret and the rate of fibertake up can be varied to control production rate, fiber size, morphologyand draw ratio. Preferably, the volume rate of supply of the dope is 50to 500 cc/minute, most preferably, 100 to 300 cc/minute.

The spinnerets employed in the process of the invention are maintainedduring extrusion at a temperature sufficient to attain a viscosity ofthe spin dope sufficient to facilitate draw down of the nascent fiber.Generally, the spinneret may be maintained at 40° to 130° C., preferably60° to 100° C.

During extrusion of one polymer solution through a hollow fiberspinneret, a bore fluid is injected within the bore of the fiber tofacilitate generation of the hollow fiber configuration. The bore fluidcan be a mixture of a solvent and a nonsolvent for the polymer toprovide a slow rate of coagulation and to permit draw down of the fiber,or it can be an inert gas such as N₂. Suitable bore fluids include, butare not limited to, water, N-methylpyrollidone (NMP), dimethyl formamide(DMF), and dimethyacetamide (DMAc). Preferably, the bore fluids includemixtures of solvents such as DMAc, NMP, DMF, and the like with water.

At the exit of the spinneret, the nascent fiber is briefly exposed to anair gap of a gaseous atmosphere immediately prior to contacting a fluidcoagulation bath. The choice of pressure, temperature, composition ofthe atmosphere, as well as the time period of exposure of the fiber tothe gaseous atmosphere are chosen to control the morphology of thenascent fiber.

Typically, the nascent fiber travels through the air-gap at roomtemperature. The temperature of the air gap medium can be varied tofacilitate evaporation of the solvent from the nascent fiber. Generally,the air gap may be at ambient, as well as elevated temperatures.Preferably, the air gap temperature is at ambient temperature.

The composition of the gaseous atmosphere of the air-gap is generallychosen to facilitate evaporation of the solvent from the fiber. Possiblegas compositions include, but are not limited to air, nitrogen, inertgases such as He, Ar, Xe and the like. Alternatively, pressures belowatmospheric may be employed in the air gap. Preferably, air and inertgases can be employed in the air gap. Most preferably, air is employedin the air gap.

After contacting the gaseous atmosphere of the air gap, the fibers arepassed into a coagulation bath to coagulate the fiber by extraction ofthe solvent prior to being wound onto a takeup roll. The choice of bathcomposition and temperature is made to control the rate of coagulationand morphology of the fiber. Possible compositions of the coagulationbath that may be employed in the invention include, but are not limitedto water, aliphatic alcohols, mixtures of aliphatic alcohols, andmixtures of aliphatic alcohols with water. Other possible compositionsfor the coagulation bath include aqueous solutions of DMF, NMP, andDMAc. Preferably, the composition of the coagulation bath is a mixtureof aliphatic alcohols and water. Most preferably, the bath compositionis water. The temperature of the coagulation bath can be varied tocontrol the rate of coagulation and fiber morphology. Generally, thebath is maintained at a temperature of less than 25° C., preferably 0°to 20° C., most preferably 5°-15° C. Suitable coagulation bathcompositions for the nascent membranes vary depending on the compositionof the polymer solutions employed and the results desired. Generally,the coagulation bath medium is miscible with the solvent or the solventmixture of the spin dope, but is a non-solvent for the polymers.However, the coagulation bath may be varied to achieve desiredproperties in the individual layers of a composite membrane. For examplefor a multicomponent membrane, the solvent of the separating layerpolymer solution may be less miscible in the coagulation bath than thesolvent of the substrate layer polymer solution allowing different ratesof solvent extraction. A coagulation bath, therefore, may be amulticomponent mixture of water and an organic solvent that is misciblewith water and the solvent to be removed from the polymer. Thetemperature and composition of the bath also may be controlled to affectthe extent and rate of coagulation. After treatment of the fiber in thecoagulation bath the fibers are wound onto a takeup roll or othersuitable collection device.

The ratio or the drawing speed of the fiber to the extrusion velocity ofthe fiber may be varied over wide limits. Generally the rate ofextrusion velocity of the fiber may vary from 2 to 100 meters/minute,preferably 3 to 50 meters/minute, most preferably 5 to 20 meters/minute.Similarly, the rate of drawing of the fiber may vary from 5 to 500meters/minute most preferably 50 to 150 meters/minute.

The nascent membrane is optionally dried under specified conditions andthen precipitated in a coagulating bath that is a non-solvent for thepolymer, but is a solvent of the polymer solvent. The nascent film canbe optionally dried at from 10° C. to 200° C., preferably 25° C. to 100°C., for 0.01 to 10 minutes preferably for 0.05 to 1.0 minutes, bypassing the nascent film through an oven. The nascent film is thenprecipitated in the coagulating bath.

The resulting fiber membranes are washed to remove residual solvent andthe like, whereafter they are dried. Typically washing is accomplishedby placing the fiber membranes into water at 25° C. to 100° C.preferably 25° C. to 75° C. for a period sufficient to removesubstantially all residual solvent as well as other impurities such asresidual additives in the spin dope. Thereafter, the fibers are airdried or dehydrated by solvent exchange. For example, the polyaramidefibers may be dehydrated by a two step solvent exchange dehydrated byfirst using methanol and then FREON® 113. Such methods of solventexchange, are known in the art, as described in U.S. Pat. Nos.4,080,743; 4,080,744; and 4,120,098. Alternatively, the fibers may bedehydrated by heating in atmosphere; such as air, and the like.

Typical solvents for the polymer solutions included solvents such asdimethyl formamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethylsulfoxide and the like. These solvents are useful with the preferredpolymer materials of the present invention, that is polysulfone,polyethersulfone, polyamide, polyimide and polyetherimide. Thesesolvents, however, are merely illustrative and should not be consideredlimiting.

Mixtures of solvents also may be used in the polymer solutions employedto form the layers of the membrane. The specific mixture of solvents mayvary. For example, two or more solvents may be used which vary involatility or solvation power.

The solvent mixture also may contain additional components such aspolymer swelling agents, and nonsolvent components. These addedcomponents may be useful, for example, to achieve a desired anisotropyin the morphology of the dense separating layer of the membrane bymoving the polymer solution closer to its point of incipient gelation.These additional components may be characterized as extractable ornonextractable in the coagulation bath. Extractable components, that is,materials which are extractable in an aqueous-based coagulation bath,may be useful, for example, as pore formers in a layer. Examples ofextractable components include inorganic salts, and polymers such aspolyvinyl pyrrolidone. Examples of additional components which may beemployed include, for example, discrete monomeric materials which areinsoluble in the composition of the coagulation bath, polymerizablematerials such as moisture-curable siloxanes, and compatible ornon-compatible polymers. The foregoing examples of additional componentsare merely illustrative and should not be considered limiting. Thefabrication processes employed to form the multicomponent membranes ofthe present invention generally depend on the major component of themembrane. For example, in manufacture of bicomponent hollow fibermembranes, selection of the spinning parameters depends on thespinability of the substrate layer solution. This means that bicomponentmembranes formed by the present invention readily can be spunessentially under the same conditions as the underlying substrate layer.

Improved Properties

A surprising advantage provided by the present invention is its abilityto produce asymmetric or multicomponent membranes having improved gasseparation properties. It is believed that the advantageous propertiesof the present gas separation membranes are derived from the decreasedfree volume of the membrane for minimizing the dual mode competition forsorption and transport in the glassy polymer matrix. Free volume is thevolume in a dense film which is available for gas sorption andtransport. Free volume may be calculated by subtracting the volumeoccupied by vibrating macromolecules from the total macroscopic volume.High flux membranes possessing high free volume were found to be moreprone to dual mode competition which depresses the mixed gas selectivityin applications such as natural gas sweetening (CO₂ /CH4) andseparations of carbon dioxide from nitrogen or air. The thickness of themembrane separating layer possessing reduced free volume is controlledto obtain the desired level of productivity.

In the prior art, others have sought to increase the free volume of gasseparation membranes, as a means of increasing membrane productivity. InU.S. Pat. No. 4,880,441 Kesting et al. state that,

"Until the present invention, only two options were available toincrease the free volume in a given hydrophobic glassy polymer utilizedfor gas separations. First, membrane density can be decreased and freevolume increased through swelling the film or membrane by exposure toCO₂ under pressure. Carbon dioxide under pressure acts as a swellingagent and at very high pressure, it can even act as a supercriticalliquid solvent, thus the CO₂ lessens polymer-to-polymer interactionallowing the macromolecules to move farther apart from one another. Forexample, Erb and Paul, J. Membrane Sci., 8, 11 (1981) illustrated thatthe CO₂ absorption of melt-extruded polysulfone film is increased byexposure to CO₂ to 60 atm. Although not explicitly stated by Erb et al.,increased CO₂ adsorption was due to increased free volume. Secondly, thepolymer can be modified so as to inhibit close packing. Chern et al.,Materials Science of Synthetic Membranes, ACS Symposium Series 269, D.Lloyd, ed. 1985: p. 40, showed that whereas films prepared frompolysulfone which utilizes bisphenol A exhibits a P for CO₂ of 4.4 andan alpha for CO₂ /CH₄ of 28; films prepared from polysulfone made withtetramethyl bisphenol A have a P for CO₂ of 21 with an alpha for CO₂/CH4 of 68. The methyl groups in the latter polymer sterically inhibit aclose approach between neighboring chains thereby increasing free volumeand permeability."

Kesting et al. proceed to disclose an asymmetric gas separation membranehaving a graded density skin and macrovoid-free morphology comprised ofglassy, hydrophobic polymers having increased free volume.

The prior art does, however, not teach a method of extrudinghollow-fiber gas separation membranes so as to provide controlled levelof decreased free volume in the fiber separating layer morphology forobtaining high mixed gas selectivity, preferably at an economicallyviable level of productivity. In the simplest case, the invention canproduce bicomponent membranes of a separating layer and a poroussubstrate layer. The separating layer may be dense or asymmetric. Inaddition, the present invention retains the advantages of the prior artby allowing gas separation membranes to be formed from separatingmaterials which are otherwise impossible or very difficult to fabricateinto commercially useful membranes. The present invention alsoadvantageously enables the use of other membrane materials which havenot been easily fabricated into useful commercial membranes due tosolubility, solution viscosity or other rheological problems.

The membranes of the present invention possess superior gas separationproperties. The multicomponent fiber membranes formed in the presentinvention possess the superior gas separation properties of theseparating layer while maintaining the ease of fabrication of thesubstrate layer.

Utility of the Inventive Membranes

The novel membranes of the invention have use in a wide variety of gasseparations. For example, the membranes of the present invention areuseful for the separation of oxygen from air to provide enriched oxygento provide enhanced combustion, and for the separation of nitrogen fromair to provide inerting systems; in recovery of hydrogen fromhydrocarbon gas in refinery and ammonia plants; separation of carbonmonoxide from hydrogen in syngas systems; for separation of nitrogenfrom ammonia; and separation of carbon dioxide or hydrogen sulfide fromhydrocarbons.

The novel multicomponent membranes of the present invention, however,are not limited to use in gas separations. Generally, all known membraneseparations can benefit from utilizing the novel membranes describedherein. For example, the membranes may find use in reverse osmosis,microfiltration, ultra-filtration or other separations such asbioseparations that require affinity of certain components in a complexmixture with the membrane to effect efficient separations. Materialswith the required affinity generally are not easily manufactured intouseful membranes. The current invention, however, enables efficientfabrication of such membranes.

EXAMPLES

In the following examples, all temperatures are set forth uncorrected indegrees Celsius; unless otherwise indicated, all parts and percentagesare by weight.

The permeability of gases through isotropic dense film membranes isgenerally defined as the centiBarrer. A centiBarrer is the number ofcubic centimeters of gas permeated by the membrane at standardtemperature and pressure multiplied by the thickness of the membrane incentimeters divided by the time in seconds for permeation and thepartial pressure difference across the membrane in centimeters of Hg,that is ##EQU1##

The flux of gases through an asymmetric membrane can be defined in termsof gas permeation units, GPU's, as ##EQU2## wherein the units aredescribed above.

Comparative Example 1

As taught in U.S. Pat. No. 5,085,676, a substrate solution containing31% total weight of a polymer blend comprising 90:10 weight ULTEM® 1000(commercially available from General Electric Co.) polyetherimide andMATRIMID® 5218 (commercially available from Ciba Geigy Corp.) polyimideand 2.3% weight LiNO₃, 9.3% weight tetramethylenesulfone, 1.6% weightacetic anhydride and 0.3% weight acetic acid in N-methylpyrrolidone wascoextruded at a rate of 104 cm³ /hr through a composite fiber spinneretwith fiber channel dimensions of outer diameter equal to 559 microns(5.59×10⁻⁴ m) and inner diameter equal to 254 microns (2.54×10⁻⁴ m) at80° C. A separating polymer solution containing 26% weight MATRIMID®5218 polyimide, 7.8% weight tetramethylenesulfone, 1.3% weight aceticanhydride and 0.26% weight acetic acid in N-methyl-pyrrolidone wascoextruded at a rate of 11.9 cm³ /hr. A solution containing 90% weightN-methylpyrrolidone in H₂ O was injected into the bore of the compositefiber at a rate of 46 cm³ /hr. The nascent filament traveled through anair-gap length of 4 cm at room temperature into a water coagulant bathmaintained at 27° C. and was wound up at a rate of 90 meters/min. Thespin draw ratio defined as the take-up velocity to the average extrusionvelocity for this example was calculated to be 9. The water-wet fiberwas washed with running water at 50° C. for about 12 hours anddehydrated as taught in U.S. Pat. Nos. 4,080,744 and 4,120,098. Thisspecifically involved the replacement of water with methanol followed bythe replacement of methanol with normal hexane and drying in a vacuumoven (2.67 kPa).

The fibers were treated to seal defects protruding through the denseseparating layer as taught in U.S. Pat. No. 4,230,463. This treatmentspecifically involved contacting the outer surface of the fibers with ahexane solution containing 1% weight SYLGARD® 184 polydimethylsiloxane.The exposure time of the fibers to each step of the posttreatment was 30minutes at room temperature and a vacuum (2.67 kPa) in the fiber bore.

The fibers were tested for mixed gas CO₂ /N₂ (10/90 mole) while applying175 psi on the shell side of the fibers at 27° C. The results arereported below:

CO₂ Productivity=110 GPU

CO₂ /N₂ Selectivity=17

The same fibers were tested for single gas CO₂ and N₂ permeationproperties at room temperature. Results are reported below:

Single Gas CO₂ Productivity=298 GPU

Single Gas N₂ Productivity=8.5 GPU

Single Gas CO₂ /N₂ Selectivity=35

The ratio of mixed gas selectivity to single gas selectivity is only

Example 1

Composite fibers were spun by using the same substrate solutiondescribed in Comparative Example 1 with a separating polymer solutioncontaining 28.5% weight MATRIMID® 5218 polyimide, 8.6% weighttetramethylenesulfone, 1.4% weight acetic anhydride and 0.29% weightacetic acid in N-methylpyrrolidone. The substrate and the separatinglayer solutions were coextruded through a composite fiber spinnerethaving the same fiber channel dimensions as described in ComparativeExample 1 at a rate of 216 cm³ /hr respectively at 97° C. A solutioncontaining 90% weight N-methylpyrrolidone in H₂ O was injected into thebore of the composite fiber at a rate of 48 cm³ /hr. The nascentfilament traveled through an air-gap length of 7.5 cm at roomtemperature into a water coagulant bath maintained at 10° C. and waswound up at a rate of 80 meters/min. The spin draw ratio for thisexample was calculated to be 3.9. The water-wet fiber was washed anddehydrated as described in Comparative Example 1.

The fibers were treated to seal defects as described in ComparativeExample 1 and tested for mixed gas CO₂ /N₂ (10/90 mole) while applying175 psi on the shell side of the fibers at 27° C. The results arereported below:

CO₂ Productivity=50 GPU

CO₂ /N₂ Selectivity=30

The same fibers were tested for single gas CO₂ and N₂ permeationproperties at room temperature. Results are reported below:

Single Gas CO₂ Productivity=97 GPU

Single Gas N₂ Productivity=2.7 GPU

Single Gas CO₂ /N₂ Selectivity=36

The ratio of mixed gas selectivity to single gas selectivity is 83%.

Comparative Example 2

A solution containing 27% weight MATRIMID® 5218, 5.4% weightTHERMOGUARD® 230 (a brominated epoxy resin commercially available fromM&T Chemical, Inc.), 1.4% weight acetic anhydride and 0.3% weight aceticacid in N-methylpyrrolidone was extruded through a spinneret having thesame fiber channel dimensions as described in Comparative Example 1 at arate of 200 cm³ /hr at 75° C. A solution containing 85% weightN-methylpyrrolidone was injected into the bore of the fiber at a rate of50 cm³ /hr. The nascent fiber traveled through an air-gap length of 14cms at room temperature into a water coagulant bath maintained at 6° C.and was wound up at a rate of 68 meters/min. The spin draw ratio forthis example was calculated to be 4. The water wet fiber was washed anddehydrated as described in Comparative Example 1. The fibers wereposttreated to seal defects as described in Comparative Example 1 andtested for mixed CO₂ /N₂ at 25° C. and 175 psig. Results are reportedbelow:

CO₂ Productivity=71 GPU

CO₂ /N₂ Selectivity=20

The same fibers were tested for single gas CO₂ and N₂ permeationproperties at room temperature. Results are reported below:

Single Gas CO₂ Productivity=162 GPU

Single Gas N₂ Productivity=4.2 GPU

Single Gas CO₂ /N₂ Selectivity=39

The ratio of mixed gas selectivity to single gas selectivity is only51%.

Example 2

This example describes an embodiment of the invention for the asymmetrichollow fibers.

A polymer solution containing 29% weight MATRIMID® 5218 polyimide, 8.7%weight tetramethylenesulfone, 1.5% weight acetic anhydride, 3% weightacid and 0.9% weight LiCl in N-methylpyrrolidone is extruded through aspinneret having the same fiber channel dimensions as described inComparative Example 1 at a rate of 200 cm³ /hr at 97° C. A solutioncontaining 90% weight N-methylpyrrolidone in H₂ O was injected into thebore of the fibers at a rate of 50 cm³ /hr. The nascent filament travelsthrough an air-gap length of 7.5 cm at room temperature into a watercoagulant bath maintained at 6° C. and was wound up at a rate of 55meters/min. The spin draw ratio for this case was calculated to be 3.2.The fibers were washed and dehydrated as described in ComparativeExample 1 and posttreated by the procedure of Comparative Example 1 andtested for mixed gas CO₂ /N₂ (10/90 mole) while applying 175 psig on theshell side of the fibers at 27° C. The results are reported below:

CO₂ Productivity=54 GPU

CO₂ /N₂ Selectivity=31

The same fibers were tested for single gas CO₂ and N₂ permeationproperties. Results are reported below:

Single Gas CO₂ Productivity=90 GPU

Single Gas N₂ Productivity=2.3 GPU

Single Gas CO₂ /N₂ Selectivity=39

The ratio of mixed gas selectivity to single gas selectivity isadvantageously 79%.

The spinning conditions and the resulting permeation properties for themembranes produced in the comparative examples and the examples aresummarized in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                Ratio of                                                          Selectivity                                                                         Selectivity                                                                         Mixed Gas                                          Air                                                                              Coagulation CO.sub.2 /N.sub.2                                                                   CO.sub.2 (single                                                                    Selectivity                              % Solids                                                                           Spinneret                                                                          Gap                                                                              Bath Temp.                                                                           Take-up                                                                            (mixed                                                                              gas)/N.sub.2                                                                        to single gas                     Example                                                                              Content                                                                            (°C.)                                                                       (cm)                                                                             (°C.)                                                                         (M/min)                                                                            gas)  (single gas)                                                                        selectivity                       __________________________________________________________________________    Comparative                                                                          26   80   4  27     90   17    35    49%                               Example 1                                                                     Example 1                                                                            28.5 97   7.5                                                                              10     80   30    36    83%                               Comparative                                                                          27   75   14  6     68   20    39    51%                               Example 2                                                                     Example 2                                                                            29   97   7.5                                                                               6     55   31    39    79%                               __________________________________________________________________________

What is claimed is:
 1. A process for making a multicomponent gasseparation membrane comprising the steps of:a. dissolving first polymeror polymers in a suitable solvent to form a core solution; b. dissolvingsecond polymer or polymers in a solvent to form a sheath solution havingat least 27-35 weight percent of said second polymer or polymers; c.coextruding the core and sheath solutions at a temperature of 85°-100°C. through a spinneret having at least one hollow fiber spinning orificeto provide at least one nascent multicomponent hollow fiber membrane; d.drawing said nascent multicomponent hollow fiber membrane through an airgap of 7-9 cm at a draw ratio of 3.0-6.0; e. introducing said nascentmulticomponent hollow fiber membrane into a coagulation bath at atemperature of 0°-20° C. to solidify the nascent multicomponent hollowfiber membrane into a hollow fiber membrane.
 2. A process for making anasymmetric gas separation membrane comprising the steps of:a. dissolvingpolymer or polymers in a solvent to form a sheath solution having 27-35weight percent of said polymer or polymers; b. extruding the solution ata temperature of 85°-100° C. through a spinneret having at least onehollow fiber spinning orifice to provide at least one nascent hollowfiber membrane; c. drawing said nascent hollow fiber membrane through anair gap of 7-9 cm at a draw ratio of 3.0-6.0; d. introducing saidnascent hollow fiber membrane into a coagulation bath at a temperatureof 0°-20° C. to solidify the nascent hollow fiber membrane into a hollowfiber asymmetric membrane.